1. Field of the Invention
The present invention relates to an image forming apparatus which acquires a toner image by developing an electrostatic latent image formed on a latent image carrier such as a photosensitive drum and then transfers the toner image on a sheet of paper. More particularly, this invention relates to an image forming apparatus in which a process unit for forming an image is constituted as an exchangeable cartridge.
2. Description of the Related Art
Image forming apparatus, such as a copying machine, a printer and a facsimile machine, employ a latent image forming type recording apparatus like an electrophotographing apparatus, due to a recent demand for image recording on normal sheets of paper. According to this image forming principle, after a photosensitive drum as a latent image carrier is precharged, the photosensitive drum is exposed to a light image to have an electrostatic latent image formed thereon. This electrostatic latent image is developed by a developing unit so that a toner image is formed on the photosensitive drum. This toner image is then transferred onto a sheet of paper.
The image forming process unit should be exchanged with a new one since the photosensitive drum has a limited service life and the developer in the developing unit will eventually run out. To facilitate the exchanging operation, the photosensitive drum, the developing unit, etc. are installed in a single cartridge so that they can be handled cartridge by cartridge.
There is a demand for a more compact image forming apparatus incorporating such a process cartridge. There is also a demand for a process cartridge which has a longer service life and is easy to exchange.
FIGS. 1A and 1B are explanatory diagrams of prior art. As shown in FIG. 1A, the image forming apparatus has a process cartridge 160 provided above a sheet cassette 170 for retaining sheets of paper, and a stacker 173 disposed above this process cartridge 160. A U-shaped feeding path 171, which extends from the sheet cassette 170 and reaches the stacker 173 through the process cartridge 160, is formed so that sheets are conveyed along this path 171. This arrangement can prevent the sheet cassette 170 from protruding from the apparatus, thus making it possible to design the apparatus compact. Further, the sheet feeding path 171 can be made short, allowing the apparatus to become more compact. As the feeding path 171 runs vertically, it is easy to remove a jammed sheet.
The process cartridge 160, which is provided in the thus constituted image forming apparatus, has a photosensitive drum 161, a precharger for precharging the photosensitive drum 161, a developing unit for developing an electrostatic latent image on the photosensitive drum 161 with a toner, and a cleaner for removing and collecting the residue toner on the photosensitive drum 161. This process cartridge 160 is exchangeable cartridge by cartridge. Provided at the rear portion of the process cartridge 160 is an optical unit 163 for exposing the photosensitive drum 161 to a laser beam to form an electrostatic latent image thereon. A transfer roller 162 is disposed opposite the photosensitive drum 161, and discharge rollers 172 are provided on the discharge side of the U-shaped feeding path 171.
A rotatable front cover 180 is provided at the front of the image forming apparatus. As shown in FIG. 1B, this front cover 180 is opened to allow the user to remove a jammed sheet. With the front cover 180 open, the process cartridge 160 can be pulled out so that it can be exchanged with a new process cartridge 160. This method is advantageous because the sheet cassette 170 can be loaded or unloaded from the front side of the apparatus (on the right-hand side of the drawing), and the removal of a jammed sheet and the exchanging of the process cartridge 160 become possible when the front cover 180 is opened.
As the apparatus becomes compact, the front cover 180 cannot have a large open angle. To exchange the process cartridge 160, therefore, the process cartridge 160 should be lifted up obliquely as shown in FIG. 1B, making the exchanging operation troublesome. As the open angle of the front cover 180 is small, exchange is difficult unless the process cartridge 160 is made smaller. Naturally, individual process units of the process cartridge 160 should become smaller. As a result, the amount of the retainable developer may become smaller, reducing the maximum number of prints allowable by a single process cartridge 160 and thus resulting a shorter exchanging cycle. Further, if the developer runs out, the process cartridge itself should be exchanged, so that the other process units should be exchanged at the same time before their service lives actually expire. This way, resources are wasted and the process cartridge 160 should be exchanged frequently.